Went into the local repco after a set of NGK BP6EFS plugs for my 300TE however they told me they had been superseded by a R type plug so I bought the BP6EFS-13 plugs instead. However the another auto store had the BP6EFS in stock so be weary which store you're at to make sure you get the non-resistor copper plugs if they're the ones you want.

I bought all remaining new old stock champion and bosch plugs factory specified for my australia version m104.980 and will continue to do so as they show up. One will have to pry them from my cold dead hands if they wanted any.

Just got a pack of 36 NGK BP6EFS plugs from an Australian eBay seller for $42 post paid. Yes 36 plugs for $42! Picked them up from a local Woolworths Supermarket parcel collection point. Should keep me (and the car) going for a bit. Unfortunately I got the last of them from him.

OK, now for the bad news. Just changed the plugs on the 300E 2.6 I drove down from Queensland for months back. Original owner and just 152,000K, and at a very good price but uncertain maintenance over the past decade using any old garage that was handy. I figured the great body and interior was worth any hassles that I found. Mostly the problems have been lack of preventative maintenance and easily fixed.
Just pulled the plugs that were in the car - Bosch FR7DC4, totally wrong plugs, square seats, not tapered and with a washer. Five changed out OK but at No4 cylinder, I can't tighten the new correct NGK plug up; the thread seems damaged in some way. The plug is reasonably tight, but if pressed any further it just keeps turning round. Looks like I'm going to be up for a head job. I'll probably take the the head off my parts car and do it up in my own (slow) time. Alternatively, anyone know if its possible to repair the damaged thread without taking the head off?
In any case, an object lesson to use the correct parts! Using the correct plugs also immediately fixed the slight hesitation when starting off and acceleration is much better. A very pronounced improvement!

Last edited by gjlockyer on Mon 12 Feb, 2018 6:03 pm, edited 1 time in total.

Thank you one and all for your help. I thought that the head had to be off for the helicoil procedure. I'll follow it up and get it done asap. The plug has not come loose after 200 k's around town. I've taken it out and wiped off most of the small dab of applied copper grease - I wouldn't mind if the plug seized up a bit! but I will get an experienced mechanic to put a helicoil in as I don't think I'll attempt it myself.

Last edited by gjlockyer on Mon 04 Dec, 2017 8:53 pm, edited 1 time in total.

Now some good news, but a mystery! After a lengthy run I checked the suspect plug to make sure it was still as tight as I could get it - and it seated home perfectly! Couldn't believe it, so I took it out entirely and then put it back, again to the correct torque. No sign of it refusing to fully tighten up and keep turning. I'll keep checking it for a bit, but it seems that it has fixed itself. Can't explain it; beyond my experience (and I'd love a dollar for every plug I've ever changed), but I'm smiling!